Exposure timer for x-ray apparatus



Feb 9, 1954 s. A. JOHNSTON 2,668,909

EXPOSURE TIMER FOR X-RAY APPARATUS Filed April lO, 1950 Patented Feb. 9, 1954 EXPOSURE TIMER FOR X-RAY APPARATUS Samuel A. Johnston, Walworth, Wis., assignor to The George W. Borg Corporation, Chicago, Ill., a corporation of Delaware Application April 10, 1950, Serial No. 155,105

(Cl. Z50-27) 13 Claims.

The present invention relates in general to exposure timers for X-ray apparatus, such as are used for controlling the duration of X-ray exposures, and the object of the invention is a new and improved timer of this character.

A feature of the invention is an electronic timer in which a time interval is measured by the dis charge of a condenser through a resistance the value of which may be preadjusted in accordance with the desired duration of the time interval.

Other features relate to circuit arrangements designed to protect against over exposure or failure of various equipment items to operate in the intended manner.

The invention will be described more fully in the ensuing specification with reference to the accompanying drawing, which is a conventional circuit diagram showing an X-ray installation embodying the invention.

Referring to the drawing, the reference characters I and 2 indicate a single phase power line,

supplying commercial alternating current at a voltage of about 115 volts. The reference character SI indicates a switch for connecting the line through to the conductors 3 and 4 and 5 and 6 over which alternating current is supplied to the apparatus. S2 is the X-ray exposure switch.

The X-ray tube is indicated at I and may be of any suitable type. Anode current for the X- ray tube is supplied by the high tension transformer I2 the primary winding of which may be energized from the auto-transformer I3 which is bridged across the conductors 5 and 6. The connections between the primary winding of transformer I2 and the auto-transformer I3 are normally open and may be closed by energization of the relay I4.

The lament current for the X-ray tube is supplied by the transformer I I, under control of the milliampere selector switch, labeled M A. selector in the drawing. The milliampere selector is a manually adjustable rotary switch having a contact arm I5, a plurality of contacts adapted to be engaged successively by rotation of the contact arm, and a plurality of resistors such as I9 which connect these contacts. The milliampere selector is bridged across the conductors 5 and 6 in series with the primary winding of the transformer I I and is used in knownmanner to adjust the load on the X-ray tube prior to making an exposure. As shown in the drawing the load may be varied from 25 milliamperes to 250 milliamperes, but the arrangement shown is merely illustrative and may be changed when necessary or desirable.

The milliampere selector is preferably provided with additional contact arms I5, Il, and I8 and an associated set of contacts the purpose of which will be explained. The contact arms Iii-I 8 are mechanically connected to contact arm I5, as

by mounting them on the same shaft, so that all said contact arms rotate together.

The electronic timer includes a timer selector switch, labeled Time selector in the drawing, which is a manually adjustable rotary switch having a contact arm 3Q and a plurality of contacts associated therewith as shown. These contacts are connected by the resistors 32-35. The timer also includes the condenser 36 which may have a capacity of 4 microfarads. The condenser has a charging circuit including the resistor 48 and a discharging circuit which includes the resistor 3l and one or more of the resistors 32-35, depending on the adjustment of the time selector switch. These circuits will be explained more fully hereinafter.

The reference character 38 indicates a space discharge device which may be a type 1J6 triode. Heater current for the cathode of the tube is supplied by the transformer 39, the primary winding of which is bridged across conductors 5 and 3.

The reference character indicates a rectifier the purpose of which is to provide direct current for the anode circuit of tube 3B and for the charging circuit of the condenser 36. The conductors 6 and 'I may be considered as the direct current output conductors and the rectifier is so connected that the conductor 'I is positive with respect to conductor 6. The condenser 41 is connected between the conductors 6 and 'i and functions to maintain a more or less ripple free positive potential on conductor 1. For this purpose it should have a fairly large capacity, on the order of 10 to 20 microfarads.

The resistors 42 and 44 and the potentiometer 43 are bridged across the conductors 5 and I and are provided to complete the cathode anode circuit of tube 38. For this purpose the slider of the potentiometer is connected to the mid point of the secondary winding of the transformer 39. The potentiometer 43 also makes it possible to adjust the cathode potential to the proper value.

Relay 40 is a direct current relay connected in the anode circuit of tube 38. Relay 4I may be any suitable type of relay adapted to operate on alternating current, as is the relay I4 previously mentioned.

The grid resistor 31 should be of very high resistance, preferably on the order of 20 megohms. Resistors 45 and 48v are small protective resistors which may have Values of about and 200 ohms, respectively.

The operation of the timer and associated parts will now be explained, it being assumed for this purpose that a picture is to be taken by means of X-rays in known manner.

When the switch SI is closed, alternating current is supplied to the conductors 3 and 4 and 5 and 6, thereby energizing the auto-transformer I3. The high voltage transformer I2 is not energized for the time being, since relay I4 is deenergized. Alternating current is also supplied to the rectifier 4f, and the condenser M :becomes charged, whereby conductor l -is ymaintained ata substantially constant positive potential with respect to conductor 6. The condenser 35 accordingly charges over a circuit Ywhich .extends trom conductor 6 by way of conductor 50, the said condenser, resistor 48, and :backscontactidfreiay Lil to conductor l. The charging Atime :is relatively very short, due to the low value oi resistor 48. It will be noted that thereis a leakage `path to conductor E by way of contact arm 30 ofthe time selector switch and resistor 3i but the :resistance of resistor 3l is so high relative to the :resistance .of resistor fli that the `cor-idenser .3s becomes charged .substantially to the potential of conductor '1.

Alternating `current is also supplied -to 'the transformer .39, which 'supplies current for .heat- -ing the cathode ,of :tube 38. Di-rect `current also flows over a pathfw-hichincludes fthe resistor 44, :the potentiometer r4.3, and the resistor 42 .and lthe drop :across theresistor 4:5 land l.the .adjacent part ,of the potentiometer 'winding "is eiiect-ive to produce a potential 'atthe cathode cftube 38 which is positive ywith :respect to conductor `l. The proper value of this potential varies with the .characteristics :of the tube 4and may be adjusted .by means of the potentiometer. :It should be high enough so that the tube will .cut oir -when :the grid potential, although necessarily lower :than the cathode potenti-al, is nevertheless ,considerably higher v than kthepotential on conductor y6. When a type '136 tube isfused the cathodepotential may be .von the order-oi about "35i/0111s.

The .condenser 36 being charged, a posi-tive potential is Aapplied :to the grid of tube .3,8 over 'a circuit which includes't'he contact arm 39 of 'the time selector switch, the `contact nn which :the

contact arm is resting, :one or more .of the v:re- 5 sistances .B2-*35, and theresistor 131. "Somegrid current can flow over the Iabove.circuit :but :the

--current is `very small .due vto the vextremely high resistance ofthe circuit as traced. This resistance also reduces the positive'potential `on the `grid to a `low value, but the :grid potential is nevertheless higher than the 'cathode potential because of the drop across the space :discharge path between the cathode and grid. :Although the .grid is therefore lpositive with respect to the cathode no current flows in the anode circuit of the tube because itis open at `contacts'of relay 4 I After the operator has closed #the switch .Sl with the lresults described, he Vwill vadjust the miiliampere selector switch in accordance with the load required by the particular .conditions under which the picture vis to 'be made. It may :be assumed in the present :case that the milli- 'ampere selector is adjusted fornmedium load Vof 1GO milliamperes -as .shown inthe drawing.

The operator may then adjust the timexselectol' lswitch in accordance vwith the .desired exposure time, which may be assumed to :bethe'minimum -timeinterval :of 1/4 second. In this case .also the switch is shown in the adjusted positionassumed.

Having adjusted the milliamperc selector switch and the timer vselector switch, and other necessary preparations having 'been made, the

`winding fof the :transformer l 2.

to the relay 4E. A positive potential is thus ap- ;plied to the anode of tube 38 `through relay 40.

current starts to `flow inthe cathode anode circuit of the tube, and relay 4G is energized. Upon en- .ergizing, relay i0 completes a circuit for relay I4,

which ,energizes .and closes the connections between the auto-transformer i3 and the primary The transformer 'l2 iiswthus'rmade :operative to supply high voltage Acurrent "to the anode circuit of the X-ray tube ,and .the ,exposure begins.

`does ,not change appreciably until the .potential -on the .condenser .becomes equal to the cathode potential.

.At this lpoint the flow of grid .current ceases-andv as thedischarge of lthe condenser'continues the Vgrid potential yfollows the ycondenser potential .and ver-y quickly becomes sufficiently negative with respect 4to the Vcathode potential to greatly reduce the anode currentfandcause relay dil to :'deenergze. IUpon fdeenergizing, relay 4l breaks the `circuit of relay t4 which .deenergizes also to interrupt the :supply of current to transformer I2 land terminate the-exposure.

The operator may now lrelease the switch S2, causing relay 4| to deenergize. This Opens the anode circuit of tube I38 and ire-establishes the charging Acircuit of condenser 3.6. Thecondenser accordingly charges again 'to jprepare for another timing operation. t will be noted that zthe Operator has to hold the switch S2 in operated position during the exposure. In the case described, Where the exposure time :is only V1A, second, the time during which 'the switch must be held operated is insignificant but it becomes appreciable `on lon-g exposures on the order of 5 seconds or more. :Relay 4l could vbe provided with a locking circuit `controlled by relay 40 if desired, .so that vonly a momentary actuation of switch .S2 would be required regardless ofthe duration of the exposure :to be made, but the arrangement shown is lpreferred on account of its simplicity and because it enables the `operator to instantly stop the exposure by merely releasing the switch incase Vanything should go wrong.

It will be understood that the resistance value ofresistor 3i is so selected that the time interval measured by the discharge of condenser 36 through this .resistor is A second, corresponding to the setting `of the time selector switch. The yValue of the `resistance of course depends on the operating `voltage and on the capacity of the condenser, but it maybe stated that inthe timer describedaresistorl havinga resistance of 47,000 ohms has been found to give the required time interval of 1/4 second.

The time interval is substantially independent of ordinary fluctuations in the supply voltage. For example, an increase in voltage which would increase the charge in condenser 3S and tend to prolong the time interval is compensated by an increase in the cathode potential which decreases the extent to which the condenser has to discharge and tends to 'shorten the time interval.

The time interval :can be increased from $4; second to any desired time interval within a considerable range by inserting vadditional resistors in the discharge circuit. This is the function of the time selector switch. The drawing shows a range up to seconds but this could be increased to about seconds if desired without increasing the capacity of the condenser 3E.

It may be pointed out, moreover, that operation over this range is accomplished without requiring the condenser to discharge more than l about two-thirds of the charge accumulated when it is fully charged. This means that the discharge curves for various resistance values are substantially linear with respect to time, that is, the condenser is never required to discharge far enough so that its discharge curve becomes essentially non-linear. The limitation on the extent of condenser discharge required is accomplished by maintaining the cathode potential at tube 38 at a relatively high value, as will readily be understood.

In view of the foregoing explanation it will be clear that the increase in resistance values required for increasing time intervals is substantially linear also. Thus the resistor 32 may have the same value as resistor 3l, and when the time selector switch contact arm is moved to its 1/2 second contact, thereby doubling the resistance in the discharge circuit, the time interval will be doubled also. When the contact arm 30 is adjusted to its 1 second contact the selected time interval is doubled again and consequently the resistor 33 should be equal in resistance to the combined resistances of resistors 32 and 3|. Similarly, resistor 34 should have a resistance equal to the combined resistances of resistors 33, 32, and 3i. For time intervals longer than 2 seconds the resistance per second has to be increased somewhat, due to a slight departure of the discharge curves from linearity and to the increasing effect of the grid current which although extremely low is nevertheless effective over a long time interval to supplement the normal discharge path of the condenser to an appreciable extent. Thus whereas the resistance value for time intervals up to 2 seconds should be about 188,000 ohms per second, this value should be increased to about 200,000 ohms per second for time intervals in the range of 3 to 5 seconds, and to 220,000 ohms per second for time intervals in the range of 6 to 10 seconds. In the timer described these resistance values have been found to give time intervals which are accurate within plus or minus 5%.

In the explanation so far it has been assumed that the milliampere selector remains adjusted for a load of 100 milliamperes while the time selector is adjusted for various exposure times in the range available. If the milliampere selector is now adjusted for a load of 150 milliamperes the contact arm i8, which rotates with contact arm I5, will short circuit the resistance 35 at the time selector switch, thereby limiting the time interval which can be selected by this switch to 2 seconds. The time selector can of course be set on its 5 second contact but such adjustment is ineiiective to introduce resistor 35 into the condenser discharge circuit because the resistor has been short circuited. Similarly, the setting of the milliampere selector for a load of 200 milyliamperes will limit the exposure time interval to 1 second by short circuiting resistors 34 and 35, while the setting of the milliampere selector for a load of 250 milliamperes will limit the exposure time interval to 1/2 second by short circuiting resistors 33, 34, and 35.

The operator is provided with information 6, which shows the duration of exposure which is permissible with each setting of the milliampere selector and will be careful not to exceed the limit in any case, but if a mistake is made the circuit arrangement described affords the necessary protection to the patient. The cross connections between the time selector switch and the milliampere selector switch which are shown are illustrative of what can be accomplished in this way and may be modified as desired. In practice both switches will generally have more contacts than are shown, giving a greater range of selection or selection by a greater number of steps and the cross connections will be arranged to provide the protection considered to be necessary in each case.

Attention is directed to the fact that -the timer circuits are so designed that if any equipment item should become defective or fail to operate properly no exposure can be made and consequently there is no danger of an over exposure due to such defect or failure. That is, the timer circuit is so arranged that if it fails at all it fails safe. For example, if the tube 38 should become defective or if any relay should fail to operate no exposure can be made. A dirty contact at potentiometer 43 will open the cathode anode circuit so that no current can flow in this circuit and relay 4i] cannot be energized. Still another example is the grid circuit of tube 38, which except for the connection through the contact arm 30 supplies a negative potential to the grid by way of resistors 3l-35 and grid resistor 31. In case the contact arm 30 should fail to make a connection in some position due to a dirty contact or to maladjustment, the discharge circuit of the condenser 36 will be open but the circuit for placing a positive potential on the grid of the tube will also be open and the tube cannot pass current because of the negative potential on its grid.

My improved electronic timer is not limited to use for timing X-ray exposures but may be used in any situation where an accurate and economical timer is required for measuring short predetermined time intervals.

While certain specific values have been given herein, it will be understood that this has been done to facilitate the explanation and that other values may be used, when selected with the proper relation to each other. It will be understood also that other modifications may be made without departing from the principles of the invention and I do not therefore wish to be limited to the precise form of the invention as shown and described herein but desire to include and have protected by Letters Patent all forms and modiiications of the invention that come within the scope of the appended claims.

I claim:

l. In an electronic timer, a condenser, a charging circuit for said condenser, a constantly closed discharge circuit for said condenser including a resistance, a timer switch for varying said resistance, means for opening said charging circuit to permit said condenser to discharge over said closed circuit, a space discharge device having a control grid, and a grid circuit connecting the positively charged terminal of said condenser to said grid and over which said condenser maintains a positive potential on said grid until it becomes discharged below a predetermined potential.

2. An electronic timer as claimed in claim 1, including means for maintaining a potential on accadde the: cathode of. said dischargey devicewhich is. a sufficient numher off volts positive to the potential onY the negatively charged terminal of, said condenser so that the dischargeoi said.condenser to4 said predetermined potential.v is substantially linear.

3i An'electronic. timer as claimed in claim 2, including a common source of potential for charging;thei-condenserandfor the cathode of said discharge;y device,v whereby an increase in. thechar.,- ing;` potential is; compensatedby an increase in the: cathodev Potential.

4.. Inian-` electronic timer,l av condenser, a chargingv circuit for saidv condenser, a discharge circuit for-said ccndenserincluding, a resistance, a timer switch for varying said resistance, a space dischargedevice havinga control g-rid and a normallyu open anode circuit, means for impressing a: potential oir the cathode of said discharge device', a switching device Whichis positive with respectto: thev potential on; the negatively charged terminal. of said condenser lor opening said charging; circuit and closing said anode circuit while said discharge circuit is closed, and a grid circuit over which"y said: condenser maintains a positive potential on said grid until it becomes discharged below.v the potential` on said cathode.

5.. A timer as claimed in claim ll, including a high resistancefin` thegridf circuit to limit the grid current to. a lowy Values,

6. A. timer asr claimed in claim 4, wherein the switching: device is self-restoring, and manually controlled means is provided for operating said switchingdevice andmaintaining it operated during the-discharge of said condenser;

7.'..In an. X-ray apparatus, a relay for controlling the duration of an exposure, a space discharge; device having'a control grid, an anode circuit for; said; device including said relay, a condenser, a charging: circuit for said condenser, a discharge circuit for said condenser including a resistance, a timer: switch` for varying said resistanca, a. grid circuit` over which the charged condenser'impresses af positive potential on said grid, means for. openingl said charging circuit to permit said condensertof discharge, means for closing saidanode. circuit' to energize said relay by current.l in said anodev circuit, and means impressing'a potential: on the cathode of said device whicnais-intermediate the potentials on the terminals of said condenser, whereby` saidrelay is deenergized whenv the condenser; becomes discharged sufficiently to reduce the grid' potential below the cathode potential.

8. In an electronictimer, a space discharge device having; a positively' charged cathode, a condenser',y a circuit for charging said condenser, a circuit including aresistance for discharging said condenser, a timer switch for varying the resistance of said discharge circuit, a circuit including said resistance for impressing. on thegrid a potential. which, is negative with respect to the cathode, and a circuit including said switch for causing said' condenser to impress a positive potential on said. grid while said, condenser is chargedabove a predetermined potential.

9. In a timer, a relay to be energized for a desiredtime interval?, aspace discharge device having a cathode anode circuitincluding said relay, said circuit being normally open, av condenser, a charging circuit for. said condenser, a time selector switch a circuit including. said switch4 connecting the positive terminal' of said condenser with the gridof said space dischargedevice, means for openlng'saijdcharging circuit and closing said cathode, anodei circuity to energize.. saidrelay, and a discharging circuit for said condenser including arvariabl'e resistance the value of which is deter-mined'. by. the setting of saidA time selector switch, whereby said, relay is deenergized after a desired. time interval by. the discharge of said condenser andthe. resulting change in the grid potentialat said space discharge device.

1.0;.. In` an. electronic tmer, a condenser, a circuit for charging said condenser, a. circuit for discharging`v said condenser, a. resistance and a timeselector includedl in one of'said circuits, said time selector beingv adjustable, to regulate the effective value cisaidresistance, a space discharge device having a control grid, and. a. circuit including said time` selector connecting one terminal of said.. condenser with said control grid and over rhich aY charge insaid condenser impresses a corresponding potential on said control grid.

1v1. Inv an electronic timer, a condenser, acircuitfor charging said condenser, a. discharge circuit for said condenser effective .upon openingsaid charging. circuit, a.. resistance, a time selector switch. adjusted to includeavarying amount of said resistance in saiddischarge circuit to regulate thedischarge rate of said condenser, a space discharge device having. a` control grid, and a circuit` including saidV time selector. connecting one terminal. of said condenser with said grid and over which saidy condenser maintains a. positivepotential` on said grid until. discharged. below a predeterminedpotential.

12. In anl electronic timer, asource; of direct current, ay condenser, a.V charging circuit connecting said condenser across'said source, a time selection switch comprising a4 progressively movable Contact arm and a plurality of ixedv contacts operativelyrelatedfthereto, a high resistance having aplurality of tapsiconnectedto-said contacts, respectively, a space discharge device, a

control circuit connecting.l the; positively charged terminal ofl saidcondenser to the grid, of saidrdevice, saidtcontrol circuit including the contact arm of said switch, a lixedfcontact engaged thereby, and a portion of said resistance, andv a dicharge circuit for. saidl condenser includingV said contact arm and contactand another portion of said resistance.v

13. In an electronic timer, a source of direct current, a condenser, a charging circuit connecting saidcondenser across said source, a voltage divider bridged across saidy source, a space discharge device having its cathodeI connected to a point on said divider, a connection between the positive terminal.` of said condenser and the grid of said device,- switching means fory opening said charging'circuitiand connecting the anodeo said device to theI positiveI terminalI or said source, thereby establishing current' iiow through said device, a relay energized' by saidcurrent now, and a circuit including. a' highv resistance through which said condenser-discharges to reduce the potential difference between the grid and cathode ofl said device, thereby reducing said current flow and deenergizing said' relay.

SAMUEL A. JOHNSTON.

References Cited in the le of' this patent UNITED STATES PATENTS Number Name Date 1,980,1464 Vlngerhoets Nov. 6, 1934 2,463,318 Schneider etal. iVlan l, 194.9 2,471,834 McDowellet al. --.rMay 31, 1949 `2,560,720 Dawson et a1'. Juiy'iv, 1951 

